The Many Ways a Star Can Die

October 13, 2015 | Sarah Tse

 The Cassiopeia A supernova
Photo credit: NASA/DOE/Fermi LAT Collaboration

Everything has a lifespan, even stars. On closer inspection, we can predict the unique fate for each star, including our own sun.

Premature Death

All stars begin as cool clouds of gas, or “stellar nurseries.” Once the cloud reaches a critical mass, it topples into gravitational collapse. The cloud folds in on itself and fragments into smaller, denser regions that release gravitational energy as heat. Eventually, the internal pressure causes a region to condense into a hot, spherical “protostar.” At this early stage, the protostar floats within a spinning disk of dust and gas called a “protoplanetary disk,” from which planets can form like lumps in porridge. The protostar continues to draw in gas and dust from the surrounding cloud until it reaches its mature mass, at which point it has become a visible “pre-main sequence” (PMS) star.

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If the PMS star has only acquired about a tenth of a solar mass by the end of its main accretion phase, it won’t reach a high enough temperature to begin nuclear fusion. These so-called “brown dwarfs” never attain the same luminosity as their more massive brethren. They may appear magenta or orange-red to the human eye, and are doomed to slowly cool down and fade away over hundreds of millions of years.

Stellar Adolescence

If they’re lucky, pre-main sequence stars reach a core temperature of about 10 million kelvin, which triggers a chain reaction of hydrogen fusion into helium. The onset of nuclear fusion could be considered a stellar puberty that radically changes the star’s chemical and energetic structures. The heat released by hydrogen fusion brings the star into thermal equilibrium, balancing its interior energy with the heat radiated from its outer shell. This heat also creates enough core pressure to support the weight of the star’s outer plasma layers, creating hydrostatic equilibrium. The star, now stable enough that it won’t experience any further gravitational collapse, can be classified as a full-fledged main sequence star.

The main-sequence phase lasts as long as it takes to fuse all the hydrogen in the star’s core into helium. The smallest, coldest stars, called “red dwarfs,” fuse hydrogen at a steadfast pace for 6-12 trillion years. Since the observable universe is only 13.8 billion years old, astronomers still aren’t sure what happens to red dwarfs after they run out of hydrogen.

Slightly more massive stars burn more quickly, so we can observe the aftermath. When the core burns through its hydrogen, it stops generating energy and begins to contract. The layer of hydrogen directly outside the core begins burning, while the helium core heats up. The star swells into its “red giant” phase and scorches through its helium core to create carbon. Once the helium is exhausted, the core continues to contract and heat up, causing the adjacent layer of helium to burn and igniting another round of hydrogen fusion further from the core.

Death, and Beyond

What happens next depends on the star’s mass. In red giants of intermediate mass, like our Sun, the coexistence of shells burning hydrogen and helium disrupts the star’s equilibrium. The carbon core contracts until it achieves electron degeneracy pressure, wherein electrons move faster and faster to avoid each other until they create enough pressure to support the core.

Now a “white dwarf,” the star continues losing energy until it subsides into a cold “black dwarf,” although the observable universe isn’t yet old enough for any to exist. However, some white dwarfs aren’t content to fade into ignominy and steal from an adjacent star in a desperate bid to shine once more. The white dwarf ends up expelling its outer layers in a flash of light called a “nova.”

The most massive red giants truly “live fast, die young, and leave a beautiful corpse.” The core continues heating up, falling into a series of nuclear fusion reactions creating heavier and heavier elements. Finally, it reaches iron, which is too heavy to fuse of its own accord. At this point, the same gravitational collapse that sparked the star’s birth now hastens its demise. The star releases its gravitational potential energy until the core completely collapses in a “supernova,” where it disgorges its contents in a brilliant display that can shine brighter than an entire galaxy.

The collapse of the star’s core smashes electrons and protons into neutrons and neutrinos. When the neutrons are packed together as tightly as possible, they can exert enough pressure to support the star’s mass, forming an extremely small, dense “neutron star.” As they rotate, neutron stars emit periodic pulses of light every time their magnetic poles align with Earth, earning the name “pulsar.” But if the star’s core is too heavy to be supported by neutron pressure, it continues collapsing until it becomes a black hole — the densest, darkest objects in the universe.
Supernovae and novae are responsible for seeding the cosmos with all the heavier elements we see on Earth. They also send out shock-waves of stellar material and energy that can incite a nearby nebula into gravitational collapse. Thus, the cycle resumes, and a new star is born.

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